The following description of the background of the invention is provided to aid in understanding the invention but is not admitted to be prior art to the invention.
Cellular signal transduction is a fundamental mechanism whereby external stimuli regulating diverse cellular processes are relayed to the interior of cells. One of the key biochemical mechanisms of signal transduction involves the reversible phosphorylation of proteins, which enables regulation of the activity of mature proteins by altering their structure and function.
The best characterized protein kinases in eukaryotes phosphorylate proteins on the alcohol moiety of serine, threonine, and tyrosine residues. These kinases largely fall into two groups, those specific for phosphorylating serine and threonine, and those specific for phosphorylating tyrosine. Some kinases, referred to as "dual specificity" kinases, are able to phosphorylate on tyrosine as well as serine/threonine residues.
Protein kinases can also be characterized by their location within the cell. Some kinases are transmembrane receptor proteins capable of binding ligands external to the cell membrane. Binding the ligands alters the receptor protein kinase's catalytic activity. Others are non-receptor proteins lacking a transmembrane domain. Non-receptor protein kinases can be found in a variety of cellular compartments from the inner-surface of the cell membrane to the nucleus.
Many kinases are involved in regulatory cascades where their substrates may include other kinases whose activities are regulated by their phosphorylation state. Ultimately the activity of a downstream effector is modulated by phosphorylation resulting from activation of such a pathway.
The serine/threonine kinase family includes members that regulate many steps of signaling cascades, including cascades controlling cell growth, migration, differentiation, gene expression, muscle contraction, glucose metabolism, cellular protein synthesis, and regulation of the cell cycle.
An example of a non-receptor protein kinase that phosphorylates protein targets on serine and threonine residues is RAF. RAF modulates the catalytic activity of other protein kinases, such as the protein kinase that phosphorylates and thereby activates mitogen activated protein kinase (MAPK). RAF itself is activated by the membrane anchored protein RAS, which in turn is activated in response to ligand activated tyrosine receptor protein kinases such as epidermal growth factor receptor (EGFR) and platelet-derived growth factor receptor (PDGFR). The biological importance of RAF in controlling cellular events is underscored by the finding that altered forms of RAF can cause cancer in organisms. Evidence for importance of RAF in malignancies is provided in Monia et al., 1996, Nature Medicine 2: 668, incorporated herein by reference in its entirety including all figures and tables.
In an effort to discover novel treatments for cancer and other diseases, biomedical researchers and chemists have designed, synthesized, and tested molecules that inhibit the function of protein kinases. Some small organic molecules form a class of compounds that modulate the function of protein kinases. Examples of molecules that have been reported to inhibit the function of protein kinases are bis monocyclic, bicyclic or heterocyclic aryl compounds (PCT WO 92/20642), vinylene-azaindole derivatives (PCT WO 94/14808), 1-cyclopropyl-4-pyridyl-quinolones (U.S. Pat. No. 5,330,992), styryl compounds (U.S. Pat. No. 5,217,999), styryl-substituted pyridyl compounds (U.S. Pat. No. 5,302,606), certain quinazoline derivatives (EP Application No. 0 566 266 A1), seleoindoles and selenides (PCT WO 94/03427), tricyclic polyhydroxylic compounds (PCT WO 92/21660), and benzylphosphonic acid compounds (PCT WO 91/15495).
The compounds that can traverse cell membranes and are resistant to acid hydrolysis are potentially advantageous therapeutics as they can become highly bioavailable after being administered orally to patients. However, many of these protein kinase inhibitors only weakly inhibit the function of protein kinases. In addition, many inhibit a variety of protein kinases and will therefore cause multiple side-effects as therapeutics for diseases.